Process for the benefication of phosphate ores



United States 2,903,131 Patented Sept. 8, 1959 PROCESS FOR Til-IE BENEFICIATIQN F PHOSPHATE ORES Charles E. Heinrichs, Richmond, Va., assignor to Virginia-Carolina Chemical Corporation, Richmond, Va., a corporation of Virginia No Drawing. Application October 19, 1955 Serial N0. 541,531

3 Claims. (Cl. 209-3) The present invention is concerned with the concentration of phosphate minerals from their ores. Specifically, it is an improvement upon the over-all process presently in use for the mining, washing and flotation concentration of phosphate rock ores.

The usual procedure for the beneficiation of phosphate ores may be divided into three steps: 1) mining, (2) washing, and (3) flotation. In most mining operations, the overburden, from to 50 feet thick, and the matrix below it, from zero to 50 feet thick and containing phosphate pebble ore, are excavated by large draglines. The dragline removes the overburden and generally disposes of it into a mined-out area. The matrix is then transferred into a small improvised sump formed in the overburden on the edge of the mine pit. A hydraulic gun supplies water to the sump and simultaneously slurries the matrix. From the sump the matrix slurry is pumped to the washer.

Phosphate washer plants embody combinations of in clined stationary screens, rotary screens, log washers, and often some type of mill or disintegrator. These are so arranged as to effect the elutriation of clay; its removal; and the desired sizing of the phosphate pebble and sand contained in the original matrix. The undersize granular discard from the washer constitutes the feed for the flotation and/ or tabling or spiral concentration plant.

Such flotation feed comprises phosphate and sand particles from about -14 mesh down to about +200 mesh. Conventional equipmentof the type of hydroseparators or cones is employed to deslime this granular washer discard material. The deslimed product is then sized in conventional sizing equipment, as for example Fahrenwald sizers. The major fraction ofabout -35 to about +150 mesh goes to the flotation cells and the about l4- to about +35 mesh fraction goes to the agglomeration concentration units.

A typical flotation process is illustrated in Patent No. 2,293,640. This procedure, which includes the so-called double float, is used in all of the examples employed as illustrations of the practice of my invention. However, although not specifically illustrated, the practice of my invention with the single, or fatty-acid float, will also yield superior results when compared to the same process not embodying my invention.

, Particles coarser than about 14 mesh, called washer rock, comprise anywhere from 15 to 85% of the granular phosphate content ofthe matrix, depending on the nature of the deposit. Conversely the particles smaller than about 14 mesh, which make up the flotation plant feed, may also comprise anywhere from 15 to 85% of the granular phosphate content of the matrix. Since the Washer rock is normally transported from the washer to storage and subsequently to drying operations, no difliculty is encountered if the supply thereof varies. However, the economical operation of a flotation plant is dependent upon a constant feed. The operation of a flotation plant directly attached to, and entirely dependent upon, the receipt of the total undersize from the washing operation does not generally receive a uniform supply thereof and is therefore generally highly ineificient, but it is the general practice employed.

This inefficiency results not only from the varying feed but also from the fact that the properties of the phosphate ore differ from one mining area to another. This variation in properties causes variations in particle size and phosphate content of the feed. These variations may be considerable in even a single days operations.

in order to operate efliciently the mine, the washer, and the flotation plant must all operate continuously at maximum capacity. This can not be accomplished in the usual procedure in view of the above-described variations in supply, phosphate content, and particle size of the flotation plant feed. In addition such variations in quantity and quality of flotation feed result in very irregular proportions of cell and table feed within the flotation plant itself. Furthermore, these variations necessitate continuous adjustment of the reagent supply to the flotation cells and agglomeration units and result in reagent waste and loss of phosphate concentrate.

The process of my invention avoids many of the difliculties outlined above. it yields a constant supply of flotation feedthus permitting the flotation and agglomeration units to operate at maximum capacity and top efficiency. The washer plant is also permitted to function at maximum capacity and peak efficiency for there is always an outlet for the undersize discard no matter what its quantity or quality. My process also provides flotation feed that is uniform in its cell and table proportions. It supplies a feed of substantially uniform quality in respect to phosphate content. Variations in such content are gradual, not abrupt, and reagent supply is adjusted smoothly.

In addition to these above advantages, an entirely new and unexpected result has been obtained by the operation of my process in that the grade of phosphate flotation concentrate produced is substantially and significantly improved.

I have discovered that if a storage area, hereinafter referred to as a conditioning area, is installed between the washer and the flotation plant that a constant quantity of flotation feed of uniform quality and of significantly constant size gradation will result. Such an area acts analogously to a mechanical fly wheel. It allows the washer to be operated at its maximum capacity without overloading the flotation plant. It also assures the flotation plant of adequate feed even in the event that the washer is. temporarily furnishing little or no undersize feed. These are obvious advantages. However, in addition I have found that when the flotation plant feed is stored for a period of at least about three weekspsubsequent flotation concentrates are of significantly improved grade (percent B.P.L.) over similar flotation concentrates produced from unstored feed. Furthermore the grade of the flotation concentrates is much more uniform showing less deviation in day-to-day operation. This results in more economical flotation reagent consumption and easier plant operation. Additional unpredictable advantages are a very significant increase in total metallurgical recovery, a decrease in the insoluble content of the final phosphate concentrate, and asubstantial increase operation of my process significantly reduces these high losses due to sliming.

A still further novel result is the uniform proportioning of table and cell feed in the flotation plant which can be brought about when the conditioning area is mined for even particle distribution.

Obviously, for all of these results to occur, it appears that some beneficial change must take place in the phosphate ore while it remains in the conditioning area. One possible explanation for the unexpected results obtained by my process concerns the transition of soluble silicates to more insoluble forms during this storage period. Phosphate ores are known to contain soluble silicates. If on exposure to air, such silicates air slake in some fashion and become less soluble, then it is conceivable that better flotation separation might be obtained. Such a mechanism for my process has not been proved and I insert it here merely as one theoretical approach.

In addition, during the operation of my process a definite hardening or cementing eifect takes place in the so-called soft or light-colored phosphate. This soft fraction of the normal phosphate ore is usually of higher grade than the darker ore fraction. It is also the most easily converted to slime during agitation either in the pump lines or in the flotation and tabling units. Since it is of high grade its loss through sliming is economically detrimental to the over-all process. The higher grade concentrate produced in my process may in part result from more complete recovery of these soft phosphates.

In the practice of my invention, the undersize yield of the washer is pumped to a large, outdoor conditioning area. Such an area, usually surrounded by earth dikes, should be large enough to hold at least several months undersize washer output. In actual practice I have used a conditioning area large enough to hold the undersize washer output from several years operation eg about 100 acres for an undersize output of from about 3000 to 7500 tons per day. Such a large size, while not absolutely necessary, has the advantage of being large enough to permit the use of a central divider so that the washer output may be deposited into one section while the conditioned material is being removed simultaneously from the other section. Of course, operation with two or more separate, smaller areas is also within the scope of my invention. However, these areas must be large enough to accommodate several weeks washer output and such output must remain in these areas for at least about three weeks.

Small surge areas have been used previously in the phosphate industry. However, these areas were operated solely for reservoir purposes. The material was stored for only a few hours, or at the most a few days, and no significant advantages, as disclosed above, from storage for several weeks duration have been reported.

In depositing the slurry of undersize from the washer, which slurry generally contains l520% solids, in the conditioning area, it may be deposited in any manner which will build up a deposit of solids, the section of which in a vertical plane extending from the point or line of discharge of the slurry to the edge of the deposit is roughly triangular or wedge shaped. Thus the slurry may be deposited at a point and flow radially in all directions forming a relatively flat cone of solids or it may be discharged from a perforated pipe and permitted to flow laterally in one or both directions perpendicular to the pipe thus forming a single or double wedge shaped deposit. A deposit may be formed which varies in depth from e.g. 30 or 40 feet at the point of discharge of the slurry to zero at the remote edge of the deposit. The size of particles in such a deposit will decrease gradually from the coarsest particles in the slurry which deposit adjacent the point or line of discharge of the slurry to the finest particles adjacent the remote edge of the deposit. It is important in forming such a deposit of undersize to maintain the surfaces of the deposit smooth and free of gutters so that the slurry as it is discharged will spread evenly over the surface of the deposit and give a fairly uniform gradation of the particle size in the deposit. It may be necessary in forming such a deposit to move the slurry discharge pipe from time to time or otherwise to smooth out irregularities in the surface of the deposit.

Many variations of this depositing technique are possible and will be apparent to those skilled in the art of disposing of aqueous slurries or suspensions of solids. Any method of depositing the slurry which gives a deposit of solids which has a uniform gradation of particle size and which permits the vehicle water carrying slime material to flow away from the deposit is within the scope of my invention.

Some of the water in the deposited slurry may evaporate or filter into and through the deposit but a substantial portion of it will flow outwardly over the edge of the deposit where it may be permitted to accumulate in a pond or it may be flowed or pumped away from the conditioning area. For the purpose of the present invention it is immaterial what is done with this water which carries slime material, provided that it is separated with the slime material from the deposit of undersize ore material. This procedure elfects an important desliming of the ore material and substantially improves the operation of the flotation part of the process.

The length of the time of storage in the conditioning areas is important. I have found that at least about 3 to 4 weeks is suflicient. In this length of time conditioning of the deposited material has progressed to such a point that substantially maximum benefits are obtained in the subsequent flotation. Additional conditioning does not materially increase the benefits over and above those which result from the first 3 to 4 weeks conditioning period and it does not decrease said benefits.

Weather and atmospheric conditions have an effect on the length of storage required. In prolonged rainy weather air slaking will take place slowly and the optimum total storage time required may exceed the average. In hot dry seasons optimum storage time may be a little less than average. I have selected at least about 3 to 4 weeks as an average, reasonable length of time for storage. Even in rainy weather storage for 3 weeks has been found to be effective.

The deposited and stored undersize material preferably is mined from the conditioning area in an orderly manner in order to provide a supply of material to the flotation step of the process. For instance one may start mining the deposit at the fine end of the deposit and proceed toward the coarse end or start at the coarse end and proceed toward the fine end. In either case the flotation operation will require only gradual adjustment to provide optimum conditions for the size of material being treated. Or the mining of the deposit may be manipulated so as to mix the coarse and fine portions of the deposit and thus to maintain a uniform feed of the material to the flotation step. All such methods of mining the conditioned fines give equivalent end results from the flotation step.

The conditioned material generally is mined by being cut down through the deposit with a hydraulic gun and the resulting slurry is pumped to the flotation plant where it undergoes the customary desliming in conventional equipment such as hydro-separators or cones. The resulting fraction of about 35 to about mesh is transported to the flotation cells and the about l4 to +35 mesh fraction goes to the tabling or agglomeration units. Such units may be illustrated by Patent No. 1,968,008 to Chapman and Littleford. The double float process of Crago, Patent No. 2,293,640, is employed for flotation separation.

Table I compares the analyses of the total of the flotation and tabling concentrates produced by present normal methods of using unconditioned floation feed and by my novel method of employing conditioned floation feed; each shown for an entire months' actual plant operation. It is readily apparent that the percent B.P.L. of concentrates from my process using conditioned feed is significantly higher (76.04% vs. 74.91%) than the concentrates from the normal process using unconditioned feed. Also the SiO content of the final concentrate is very significantly less (3.2% vs. 4.6%) for my process using con- 6 sumption, the total metallurgical recovery from the process of my invention is 1.3% larger than for the regular process. Also of particular economic significance is the increase in the average daily production. This increase amounts to 321 tons or 15% and, of course, results from the better phosphate recovery, as illustrated in Table I; from more uniform proportioning of cell and table feed; from better desliming and thus less phosphate loss in the flotation process due to the formation of slimes; and from Table III above represents 27 days plant operation during the month of September 1955. The average B.P.L. is about 0.5% higher and the results are highly consistent, the spread of these values being only 0.9%.

ditioned feed. This latter result is indicative of a much n ll r fli ie t -4 m operation. better flotation separation. I TABLE III When the results shown in Table I are examined by standard statistical methods, the standarddeviation of the V I results by my process using conditioned feed is 0.416% Conditioned with an extreme of 76.7%75.0%=1.7%. The same Day values for the process using unconditioned feed are g g g qg 1.10% and 78.8%72.6%=6.2%.- This is substantial 2 proof that my process results in flotation concentrate of 1 76 3 5 both increased, and more uniform, grade. 2 7 9 The results shown in Tables I and II were obtained by Z mining the deposit of conditioned ore so as to produce 1 from the deposit a substantially constant particle size dis- H 5% tribution in the feed to the flotation step. 8 9 g; 1 9 76. 8 2.9 TABLE I In 76.8 3.2 762 4. 0 Unconditioned Conditioned 1g 3 g: g Da it 76.2 3.4 y Percent Percent Percent Percent 16 j g 9 BPL S102 BPL S10; 17 7 2 3 5 1 76.4 4.1 73.5 5. 5 76.1 a. 3 i if 73. 4 4. 5 75. 0 2. 7 21 7 5 3 8 75. 9 3. 7 75. 8 2. 6 22 7 4 6 75. 8 3. 7 76. 2 3. 0 23 7 g 3 2 75. 5 4. 6 76. O 3. 4 24 7 3 3 7 75. 5 4. 9 76. 1 3. 2 25 7 7 3. 5 ;%-g i3 32? 35 26. 76.7 4.7 2: g 2 3 3g: 2 g: g 27 76. 7 3. 3 7 0 6 m 2 3' 1 ve e 43 4 75.8 4. 5 76.1 3. 2 Z 2 2:2 33:; Figures are averages for one months plant operation 74.3 4.8 75.7 3.6 74.6 3.9 76.0 3. 5 74. 6 4. 9 75. 0 4. 6 Reagent Total Average 74. 5 6.0 76. 2 8. 4 Oonsump- Metallnr- Daily 76. 6 4. 6 75. 6 3. 9 Flotation Feed tion, gical Produc- 75.7 4. 7 76. 3 3. 2 lbs/ton Recovery, tion, Tons 75.0 5. 1 75.0 4. 0 Percent 74. 7 4.1 76. 4 2. 9 4 73.8 3. 9 76.4 3.1 5 74. 6 4. 9 76. 1 3. 2 Conditioned 26. 62 91. 8 2, 971 74.6 5.3 75.8 3.6 74 0 5. 9 75. 4 4. 2 74.7 4. 5 76. 2 2. 8 78. 8 4. 5 7 2. 7 74.8 4.1 5 2. 7 75.6 4.6 7 2. 4 5 3. 9 2 2. 8

Average 74. 91 76. 04

TABLE 11 (Figures are averages for one months plant operation) Reagent Total Average Oonsump- Metallur- Daily Flotation Feed tion, gical Produc- 1bs./ton Recovery, tion, Tons Percent Unconditioned 29. 86 89. 9 2, 128 Conditioned 28. 11 91. 2 2, 449

1 Reagent consumption is the total amount of flotation reagents employed in the double float process per ton of product phosphate concentrate.

2 Total metallurgical recovery is the precent of phosphate recovered as compared to the actual phosphate content of the flotation feed.

Table H above illustrates three more properties which are significantly improved through the practice of my invention. The reagent consumption is reduced by about 1.75 lb./ton of product phosphate concentrate. This is a substantial saving in high-cost amines and fatty acids which together with caustic soda and fuel oil are, of course, the flotation agents employed (see US. Patent No. 2,293,640). In addition to this improved reagent con- It is obvious from the above results that operation of my process produces very significant advantages for the phosphate industry. Such advantages were entirely unpredictable. The incorporation of a conditioning and storage area could not have been predicted to produce any significant improvements in phosphate grade, total recovery, reagent consumption, and more uniform plant operation.

I claim:

1. Process of concentrating phosphate ore which comprises depositing a fraction of the ore suitable for flotation concentration in the form of a slurry in water at at least one point along a substantially straight line in a storage area, flowing the slurry away from said point at a rate permitting the solids in said slurry to deposit and form a bed of gradually decreasing particle size in the direction of flow, flowing the water carrying slime material away from said bed, interrupting the flow of slurry to said area, storing said bed in the open air for at least 3 weeks, mining material from said bed progressively from a location remote from said point toward said point and delivering the mined material to a flotation step whereby there is a gradual variation in the average particle size of the feed to said flotation step.

2. Process of concentrating phosphate or which comprises depositing a fraction of the ore suitable for flotation concentration in the form of a slurry in water at at least one point along a substantially straight line in a storage area, flowing the slurry away from said point at a rate permitting the solids in said slurry to deposit and form a bed of gradually decreasing particle size in the direction of flow, flowing the water carrying slime material away from said bed, interrupting the flow of slurry to said area, storing said bed in the open air for at least 3 weeks, mining material from said bed progressively from a location adjacent to said point toward a location remote from said point and delivering the mined material to a flotation step whereby there is a gradual variation in the average particle size of the feed to said flotation step.

3. Process of concentrating phosphate ore which comprises depositing a fraction of the ore suitable for flotation concentration in the form of a slurry in water at at least one point along a substantially straight line in a storage area, flowing the slurry away from said point at a rate permitting the solids in said slurry to deposit and form a bed of gradually decreasing particle size in the direction of flow, flowing the water carrying slime material away from said bed, interrupting the flow of slurry to said area, storing said bed in the open air for at least 3 weeks, mining material from said bed simultaneously and progressively from a location adjacent to said point and a location remote from said point toward an intermediate point in said bed, mixing the mined material and delivering the mixture to a flotation step whereby a substantially uniform feed of said material to said flotation step is maintained.

References Cited in the file of this patent UNITED STATES PATENTS McGarry Oct. 29, 1957 

1. PROCESS OF CONCENTRATING PHOSPHATE ORE WHICH COMPRISES DEPOSITING A FRACTION OF THE ORE SUITABLE FOR FLOTATION CONCENTRATION IN THE FORM OF A SLURRY IN WATER AT LEAST ONE POINT ALONG A SUBSTANTIALLY STRAIGHT LINE IN A STORAGE AREA, FLOWING THE SLURRY AWAY FROM SAID POINT AT A RATE PERMITTING THE SOLIDS IN SAID SLURRY TO DEPOSIT AND FORM A BED OF GRADUALLY DECREASING PARTICLE SIZE IN THE DIRECTION OF LOW, FLOWING THE WATER CARRYING SLIME MATERIAL AWAY FROM SAID BED, INTERRUPTING THE FLOW OF SLURRY TO SAID AREA, STORING SAID BED IN THE OPEN AIR FOR AT LEAST 3 WEEKS, MINING MATERIAL FROM SAID BED PROGRESSIVELY FROM A LOCATION REMOTE FROM SAID POINT TOWARD SAID POINT AND DELIVERING THE MINED MATERIAL TO A FLOTATION STEP WHEREBY THERE IS A GRADUAL VARIATION IN THE AVERAGE PARTICLE SIZE OF THE FEE TO SAID FLOTATION STEP. 